1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process, for example, an image forming apparatus such as a copying machine, a printer, a facsimile and the like.
2. Related Background Art
In recent years, as color image forming apparatuses of electrophotographic process type have come into wide use, not only high print quality of a color image but also high speed color output have been requested more and more. In order to meet such request, several image forming processes have been proposed.
Among such processes, there is an image forming process of so-called tandem type in which black (Bk), cyan (C), magenta (M) and yellow (Y) toner images are formed on drum-shaped image bearing members respectively and the toner images are successively transferred onto a transfer material conveyed by a transfer conveying belt by respective transfer means and then the transferred toner images are fixed to the transfer material.
An example of the conventional color image forming apparatus of tandem type is disclosed in Japanese Patent Application Laid-open No. 9-288396. FIG. 8 shows such a color image forming apparatus of tandem type.
In this color image forming apparatus, a transfer material cassette 21 is mounted to a lower part of a main body of the image forming apparatus. Transfer materials contained in the transfer material cassette 21 are picked up one by one by a pick-up roller 22 and the picked-up transfer material is fed to an image forming portion by pairs of rollers 23a, 23b. 
In the image forming portion, a transfer conveying belt (transfer material bearing member) 4 for conveying the transfer material is extended around a plurality of rollers (rotary members) 16, 17, 18, 19, so as to move in a sheet feeding direction (from right to left in FIG. 8) and, at a most upstream side of the belt, the transfer material is electrostatically adsorbed onto the transfer conveying belt 4 by an adsorbing roller portion 10 to which bias is applied. Further, four drum-shaped image bearing members (referred to as xe2x80x9cphotosensitive drumsxe2x80x9d hereinafter) 1 are arranged in a line and opposite to the conveying surface of the belt, thereby forming the image forming portion.
Around each photosensitive drum 1, and in the vicinity of the peripheral surface of the photosensitive drum 1, there are disposed a charger 2, a developing device 3, and transfer means 5 which is opposed to the corresponding photosensitive drum with the conveying surface of the transfer conveying belt 4 interposed therebetween. Bk toner, C toner, M toner and Y toner are contained in frames of the developing devices 3, respectively, in order from an upstream side (from the right). Further, between the charger 2 and the developing device 3, there is provided a predetermined gap through which exposure light from exposure means 6 is illuminated onto the peripheral surface of the corresponding photosensitive drum 1.
In this color image forming apparatus, when the transfer conveying belt 4 is rotatively moved in an anti-clockwise direction (FIG. 8) to convey the transfer material, the peripheral surfaces of the photosensitive drums 1 are uniformly charged with predetermined charges by the respective chargers 2, and the charged peripheral surfaces of the photosensitive drums 1 are exposed by the respective exposure means 6 in response to image information thereby to form electrostatic latent images on the drums, and toners from the developing devices 3 are transferred to low potential portions of the electrostatic latent images to visualize the latent images as toner images. The toner images formed on the peripheral surfaces of the photosensitive drums 1 are transferred onto the conveyed transfer material by attraction of charges created on the transfer material by transfer electric fields generated by the respective transfer means 5. The toner images transferred to the transfer material are thermally fixed to the surface of the transfer material in a fixing portion 7 comprising a pressure roller and a heating roller. Thereafter, the transfer material is discharged out of the image forming apparatus by a pair of discharge rollers 8.
By the way, in the color image forming apparatus having the plurality of photosensitive drums 1, in order to ensure that the transfer conveying belt 4 is surely contacted with the photosensitive drums 1 at the transfer portions, support rollers 9 for supporting the transfer conveying belt 4 against the photosensitive drums 1 are provided between the transfer means 5 and the transfer means 5.
Now, image forming modes will be described. The image formation is not always effected with multi-color or full-color, but monochromatic printing (printing with black color) is also effected frequently. To cope with such circumstances, there has been proposed a color image forming apparatus of tandem type in which full-color printing and monochromatic printing can be switched from one to the other.
In such an apparatus, as shown in FIGS. 9A and 9B, by integrally rotating transfer portions other than a black transfer portion together with a transfer conveying belt 11 around a support roller 12 which is contacted with an inner surface of the transfer conveying belt 11 and which is disposed at an upstream side of a black image forming portion 14 (most downstream image forming portion in a transfer material feeding direction), an advancing path extending from a transfer material adsorbing portion 13 through the black image forming portion 14 to a fixing portion 15 is not changed between a full-color printing mode (FIG. 9A) and a monochromatic printing mode (FIG. 9B), thereby providing stable conveying performance in the both modes.
However, in the color image forming apparatus described in connection with FIG. 8, although the transfer conveying belt 4 can positively be contacted with the photosensitive drums 1 by supporting the inner surface of the transfer conveying belt 4 by the support rollers 9, several protruded portions are generated on the outer surface (on an upper side on which the photosensitive drums are provided) of the transfer conveying belt 4 by the presence of the support rollers, and, at such protruded portions, the transfer material (electrostatically adsorbed at the adsorbing roller portion 10) may be separated or peeled from the transfer conveying belt 4 due to rigidity of the transfer material itself. In such a case, the transfer material may not be fed to the next transfer portion properly to cause jam of the transfer material, or, the conveying distance of the transfer material may be changed due to the peeling of the transfer material, thereby the transferring may be effected at the next transfer portion at a timing different from the proper image forming timing, which leads to misregister of color to deteriorate the image quality.
Further, since it is very difficult to eliminate eccentricity of each support roller 9 completely in the manufacturing process, the transfer conveying belt 4 is moved up and down between the transfer portions to change the conveying distance of the transfer material, thereby causing the misregister of color. Further, due to repeated stress caused by the support rollers 9, a service life of the transfer conveying belt 4 is reduced.
If the support rollers 9 are positioned not to generate the protruded portions on the transfer conveying belt 4 so as to avoid the above-mentioned phenomenon, positive and stable contact between the transfer conveying belt 4 and the photosensitive drums 1 may not be achieved.
Further, regarding members (for example, support rollers 9) contacted with the transfer conveying belt 4 between the transfer portions, an electrical condition must be devised. For example, if metallic rollers merely grounded electrically are used, transfer current will flow to the metallic rollers (and then to the earth) through the inner surface of the belt 4, thereby causing poor transferring; whereas, if the rollers are maintained to a floating condition, charges are accumulated.
Further, also regarding the image forming apparatus in which the full-color printing mode and the monochromatic printing mode can be switched and which was described in connection with FIGS. 9A and 9B, similarly, a protruded portion is generated on the transfer conveying belt at the support roller 12, which arises the above-mentioned problems regarding the conveyance of the transfer material.
An object of the present invention is to provide an image forming apparatus which can prevent misregister of color in images formed on a transfer material while effectively transferring the images onto the transfer material born on a transfer material bearing member.
Another object of the present invention is to provide an image forming apparatus which can prevent a transfer material from being peeled or separated from a transfer material bearing member.
A further object of the present invention is to provide an image forming apparatus in which images can effectively be transferred from image bearing members onto an intermediate transfer member.
The other objects and features of the present invention will be apparent from the following detailed explanation referring to the accompanying drawings.